Lebanese troops battled rebels from Syria for control of a border town Monday, the deadliest challenge in years for the armed forces of this tiny country whose own sectarian tensions could boil over from the incursion.

Dozens of armored personnel carriers, tanks and elite troops arrived to surround the town of Arsal, some 90 kilometers (55 miles) from the capital, Beirut. Meanwhile, thousands of Lebanese civilians and Syrian refugees fled the clashes in vehicles packed with all they could carry.

The fighting in Arsal, which began Saturday, marks the first time that militants battling Syrian President Bashar Assad have carried out a large-scale incursion into Lebanon. The clashes have killed 17 soldiers over three days, while 22 others remain missing after attacks on army positions in the town, authorities said.

The civilian exodus came early Monday morning during a lull in the fighting. A few hours later, an army bombardment around saw three shells land every minute.

“We call on the Lebanese army to strike with an iron fist,” said Mohammed Hojeiri, who fled Arsal with his family Monday. “Those gunmen are terrorizing civilians.”

A resident on the outskirts of Arsal told The Associated Press that the militants there committed “atrocities” and shot at people fleeing. They also looted homes, he said on condition of anonymity for security considerations.

Dozens of rebels have been killed in the fighting, he and other residents and security officials said.

It remains unclear exactly what allegiances the Syrian rebels who seized Arsal have. Their attack comes after the Lebanese army said Saturday its troops had detained Imad Ahmad Jomaa, who identified himself as a member of the al-Qaida-linked Nusra Front — one of the most powerful rebel groups fighting against Assad.

Former Lebanese Prime Minister Saad Hariri, a Sunni leader with a large following in the country, has accused al-Qaida linked groups in Syria, including the Nusra Front and the Islamic State group, of taking Arsal hostage.

The fight for Arsal is the bloodiest involving the army since the military fought a three-month battle in 2007 against the al-Qaida-inspired Fatah Islam group inside the Palestinian refugee camp of Nahr el-Bared in northern Lebanon. The Lebanese army crushed the group, but the clashes killed more than 170 soldiers.

The clashes in Arsal, a predominantly Sunni town of 40,000 whose population has almost tripled because of the presence of Syrian refugees and rebels, could worsen already bubbling sectarian tensions in Lebanon. The town is wedged between Syrian government-controlled territory and Lebanese Shiite villages sympathetic to Lebanon’s premier Shiite militia, Hezbollah.

The Syrian government, which is battling a largely Sunni insurgency, has seized nearly all the strategic Qalamoun region bordering Arsal with the help of Hezbollah fighters. On Monday, some Hezbollah fighters were seen around Labweh, a town near Arsal in Lebanon’s Bekaa Valley, though it was not clear if they were taking part in the fighting.

A senior Hezbollah official vowed Monday to support the Lebanese army against the militants.

“Whoever threatens to divide the army… we say to them that neither Lebanon nor the Bekaa (Valley) is Mosul,” said Sheikh Mohammad Yazbek, referring to Iraq’s second-largest city which has been seized by militants of the Islamic State group, which is also fighting in Syria.

But it isn’t likely that the militants in Arsal raided the town to gain territory inside of Lebanon, said Aram Nerguizian, a senior fellow at the Center for Strategic and International Studies. He said Arsal was surrounded by Christian, Shiite and even Sunni Muslim areas hostile to Islamic militants.

“This would be the definition of a very precarious deployment,” Nerguizian said.

Meanwhile in Syria, the Islamic State group shot and stabbed a family of seven to belonging to a Shiite Muslim sect to death in a rural village near the central Syrian town of Salamiyah, the Britain-based Syrian Observatory for Human Rights reported.

Syrian state media also reported the attack. The Islamic State fighters likely targeted the family because they belonged to the Ismaili sect, a branch of Shiite Islam. Many of the extremist Sunni groups fighting in Syria and now Iraq view all other sects of Islam outside their own strict interpretation as heretical.

Copyright 2014 The Associated Press